1. Field of the Invention
This invention relates to a negative-acting photoresist imaging systems, and particularly negative-acting photoresist imaging systems which are useful in non-silver halide graphic arts films.
2. Description of the Prior Art
Negative-acting photoresist imaging systems have been used commercially for many years. In these imaging systems, negative-acting photoresist compositions are applied as a layer over a metallized substrate and then subjected to actinic radiation. The actinic radiation causes a change in solubility of the layer, usually by photoinitiated polymerization of components within the layer. When the imaged layer is subsequently treated (e.g., washed and lightly scrubbed) with a developer solution, such as an aqueous alkaline solution of pH greater than 12, the non-light struck areas of the layer are washed away. If the original layer contained a colorant material (e.g., dyes or pigment), development would leave an imagewise distribution of the colored layer as a final image. If an etching or plating process is performed on the developed layer, an etched or plated image is deposited on the substrate in the exposed areas where the photoresist layer had been removed by development. A wide variety of photoresist compositions exist in the art, and many have been used as photosensitive printing plate compositions in lithographic technology. Examples of photoresist compositions known in the art include U.S. Pat. Nos. 3,395,016, 3,469,982, 3,639,185 and 4,193,797, as well as Canadian Pat. Nos. 976,352 and 976,353.
Most of the negative-acting photoresist compositions known in the art use ethylenically unsaturated materials in combination with a free radical generating photoinitiator as the active ingredients. Acrylic functional materials (i.e., acryloyl and methacryloyl functional materials) are generally preferred in the art because of their speed, toughness upon cure, and moderate cost. Printing plate compositions, in particular, rely on the use of acrylic functional materials in various forms such as monomers, oligomers and polymers with acrylic functionality. Examples of such printing plate compositions and the use of various acrylic functional materials are shown, for example, in U.S. Pat. Nos. 4,316,949, 4,228,232, 4,104,072 and 3,954,475. Many of these compositions have found acceptable commercial uses, but they have tended to be weak in certain technically important areas, particularly toughness of the cured compositions, tackiness of the uncured composition, and cold flow of the resist layer.
The most commonly used imaging systems are based on silver halide technology, diazonium salt technology, or photopolymerizable composition technology. Each of these different systems has its own advantages and problems.
Silver halide films are the most widely used graphic arts material. These films show high resolution, high imaging speeds, and consistency in their performance. However, because of the cost of silver, systems using silver halide emulsions are becoming economically disadvantageous. Other disadvantages include the requirement for multiple steps in processng, unstable processing solutions, and lack of dimensional stability.
Imaging systems using diazonium salt compositions provide high resolution and are considerably less expensive than silver to produce. The visible image produced by most diazonium salt systems consists of a dye and tends to have limits on the maximum optical densities or image color obtainable, higher minimum densities than desirable, and often suffers from a low contrast. Other disadvantages include image fade on repeated exposure, lack of dimensional stability, surface pin holes, and lack of definition at the image edge. Although thermally developable diazonium salt systems are available in the marketplace, many diazonium salt image systems still rely upon ammonia development which is undesirable because of the difficulty of working in a closed environment with ammonia. Often the diazo image comprises a dye and is not considered archival in the micrographic context.
Photopolymeric imaging systems are both inexpensive and easy to make. However, these systems usually depend upon the loading of the photopolymeric layer with opacifying material to provide optical density, which tends to reduce the sensitivity of the photosensitive layer.